1. Field of the Invention
This invention relates to a rotating head type reproducing apparatus and more particularly to an apparatus in which a recorded signal is reproduced in many recording tracks which are formed at a predetermined track pitch on a record bearing medium which is moved by a moving means, these recording tracks being traced one after another by a rotating head which is arranged to be shifted in a direction transverse to the rotating plane thereof.
2. Description of the Prior Art
In order to reproduce a stable and sharp image without noise bars being generated in carrying out so-called special reproduction which is performed at a desired reproducing speed differing from a recording speed employed, such as high speed reproduction, low speed reproduction (including still picture reproduction), reverse reproduction, etc., on a rotating head type reproducing apparatus such as VTR, one recording track must be accurately traced for each scanning field by the reproducing head of the apparatus.
To meet this requirement, a method has been known wherein a pattern signal generating device is arranged to generate a pattern signal corresponding to a distance from the scanning locus of a reproducing head obtained at an arbitrary tape travel speed to a recording track on the tape, and this pattern signal being used to control shifting mans, such as an electrical-to-mechanical conversion element (a bi-morph element, for example), which is arranged to shift the reproducing head in a direction perpendicular to the rotating plane of the head.
FIG. 1 shows a conventional VTR of the above-stated kind, and particularly, the essential parts thereof related to the present invention. A magnetic tape 1 is employed as the record bearing medium. Reproducing magnetic heads 2A and 2B are arranged at the same azimuth angle and are diametrically opposed to each other. These heads are mounted on the respective free ends of electrical-to-mechanical conversion elements 3A and 3B, such as bi-morph elements. The respective opposite ends of these conversion elements 3A and 3B are attached to a rotating member 4. The rotating member 4 is arranged to be rotated by a head rotating motor 5 in the direction of an arrow shown in the drawing. Although omitted from the illustration, the heads 2A and 2B are arranged, as well known, to be rotated in a state such that they protrude from a slit provided between a pair of tape guiding drums. Meanwhile, the tape 1 is obliquely wound, by at least 180 degrees, around the pair of drums. A rotation phase detector 6 is arranged to detect the rotation phase of the heads 2A and 2B. The output of the phase detector 6 is used as a head switch-over signal (hereinafter called the HSW signal) and is also supplied to a head motor control circuit 7. The head motor control circuit 7 is arranged to control the head rotating motor 5 on the basis of the output of the phase detector 6 via a head motor driving circuit 8 in such a manner as to cause the heads 2A and 2B to rotate at a predetermined phase and a predetermined rotational frequency. A control signal reproducing fixed head 9 (hereinafter called the CTL head) is arranged to reproduce a control signal (hereinafter called the CTL signal) recorded along the lower part of the tape at intervals of one frame distance in the longitudinal direction of the tape. A capstan 10 is arranged to form transporting means for moving the tape 1 in the longitudinal direction thereof in conjunction with a pinch roller (not shown). A capstan motor 11 turns the capstan 10. A frequency signal generator 12 is arranged to generate a frequency signal (hereinafter called the capstan FG signal) corresponding to the rotation of the capstan 10. A capstan motor control circuit 13 controls the capstan motor 11 on the basis of the CTL signal from the CTL head 9 and the capstan FG signal from the frequency signal generator 12 in such a way as to cause the capstan 10 to rotate at a predetermined phase and a predetermined rotational frequency. A pattern signal generating circuit 15 generates a pattern signal on the basis of the HSW signal from the rotation phase detector 6, the CTL signal from the CTL head 9 and the capstan FG signal from the frequency signal generator 12 in the event of reproduction at an arbitrary speed (including still picture reproduction and reverse reproduction). This pattern signal is generated for the electrical-to-mechanical conversion elements 3A and 3B and is arranged to cause each of the heads 2A and 2B to trace one recording track on the tape 1 for each scanning field. A conversion element driving circuit 16 drives the conversion elements 3A and 3B according to the pattern signal produced from the pattern signal generating circuit 15.
FIG. 2 shows, by way of example, the arrangement of the above-stated pattern signal generating circuit 15. The circuit 15 includes input terminals 17, 18 and 19 which are arranged to receive the capstan FG signal from the frequency signal generator 12, the CTL signal from the CTL head 9, and the HSW signal from the rotation phase detector 6, respectively. A binary counter 20 is arranged to count the capstan FG signal, applied to the terminal 17 and to be reset by the CTL signal applied to the terminal 18. A timing signal generating circuit 21 generates a timing signal in synchronism with the HSW signal applied to the input terminal 19. A presettable binary counter 22 is arranged to be preset by the timing signal from the timing signal generating circuit 21, with the output of the counter 20 used as preset data PD, and counts the capstan FG signal applied to the input terminal 17. A D/A converter 23 is arranged for digital-to-analog conversion of the output of the presettable counter 22 and produces a first pattern signal therefrom. The circuit 15 further includes an adder 25; an output terminal 26 to which the output of the adder 25 is applied, this output being a conversion element controlling pattern signal (or a driving signal); an oscillator 27 which is arranged to generate clock pulses of a predetermined frequency; a counter 28 for counting the clock pulses generated by the oscillator 27 and is reset by the timing signal produced by the timing signal generating circuit 21; and a D/A converter 29 for digital-to-analog conversion of the output of the counter 28. The output of the counter 22 relates to the moving speed of the record bearing medium, or magnetic tape, while the output of the other counter 28 is independent of the moving speed of the record bearing medium. The D/A converter 29 produces a still picture reproducing fixed pattern signal (or a second pattern signal).
Next, with regard to the special reproducing operation of the VTR which is arranged as described above, the operation of the VTR will be described with reference to FIGS. 3(a)-3(g), 4(A) and 4(B) attaching importance to the operation of the pattern signal generating circuit of FIG. 2. FIGS. 3(d)-3(g) show, respectively, the CTL signal produced in the event of reproduction at a speed increased by 1.5 times; the output of the counter 20 of FIG. 2; the output of the presettable counter 22 (or D/A converter 23) of FIG. 2; and the output of the adder 25. FIGS. 4(A) and 4(B) respectively show the relation of the locus of the scanning center of the heads 2A and 2B to the locus of the center of the recording tracks formed on the tape 1 obtained during still picture reproduction and during reproduction at a speed increased by 1.5 times.
The rotation phase detector 6 produces the HSW signal as shown in FIG. 3(a) accordingly as the heads 2A and 2B are rotated by the head motor 5. Then, the timing signal generating circuit 21 of the pattern signal generating circuit 15 shown in FIG. 2 produces a timing signal as shown in FIG. 3(b) in synchronism with the rise and fall of the HSW signal. The D/A converter 29 produces a still pattern signal, which is as shown in FIG. 3(c), and causes the heads 2A and 2B to continuously shift from 0 to 1 track pitch (hereinafter called TP) within the scanning range of one field.
In this instance, if the field signal, recorded on one recording track by a recording head of the same azimuth angle as that of the reproducing heads 2A and 2B, is to be reproduced alternately by the two heads 2A and 2B to carry out so-called field still reproduction, the relation of the locus of the scanning center of the heads 2A and 2B to the recording track on the tape containing the desired field, becomes as shown in FIG. 4(A). More specifically, with reference to FIG. 4(A), the full lines of the drawing represent the center loci of the recording tracks of the field signal recorded by the recording head having the same azimuth angle as that of the reproducing heads 2A and 2B. Broken lines represent those of a field signal recorded by another recording head of a different azimuth angle from that of the reproducing heads 2A and 2B. An arrow mark double line represents the center locus of scanning performed by the heads 2A and 2B. Meanwhile, a reference symbol CTL represents the recording loci of the CTL signal. (FIG. 4(B) also shows them in the same manner.) As shown, the center locus C of scanning by the heads 2A and 2B (hereinafter called the head locus) becomes a line segment diagonally connecting the beginning of the center locus a of the track to be reproduced (hereinafter called the track locus) to the end of an adjacent track locus b located on the left side of the track locus a. Therefore, in order to correct this and to adjust the head locus C to the track locus a, the following arrangement is necessary: Assuming that the travelling direction of the tape 1 is "+" and the direction reverse thereto is "-", the heads must be continuously shifted from 0 to -1 TP within the scanning range of one field.
Accordingly, in the D/A converter 29 of FIG. 2, the output of the counter 28 is converted to the still pattern signal as shown in FIG. 3(c). Then, the heads 2A and 2B can be satisfactorily shifted for the purpose of the field still reproduction.
Meanwhile, the capstan FG signal produced from the frequency signal generator 12 accordingly as the capstan 10 is rotated by the capstan motor 11 is applied to the counters 20 and 22 of the pattern signal generating circuit 15 of FIG. 2. Then, these counters 20 and 22 count the capstan FG signal. Since the counter 20 is arranged to be reset at every frame by the CTL signal coming from the CTL head 9, the highest value of the count output of the counter 20 is limited to a count value corresponding to +2 track pitches and becomes as shown in FIG. 3(e) in the event of reproduction at a speed increased by 1.5 times because the CTL signal then becomes as shown in FIG. 3(d). Meanwhile, the other counter 22 counts the capstan FG signal while the output of the counter 20 is being preset, at that point of time, by the timing signal (FIG. 3(b)) from the timing signal generating circuit 21. Therefore, the output of the counter 22 (or that of the D/A converter 23), which is produced in the event of reproduction at the speed increased by 1.5 times, becomes as shown in FIG. 3(f). Therefore, the output of the D/A converter 23 and that of the still pattern generator (counter 28 and D/A converter 24) are then added up by the adder 25. As a result, in the event of reproduction at the speed increased by 1.5 times, the adder 25 produces a pattern signal as represented by FIG. 3(g).
In the event of the 1.5 times increased speed reproduction mentioned above, the head loci of the heads 2A and 2B relative to the track loci on the tape 1 become as shown in FIG. 4(B), wherein: Reference symbols A1, A2, A3, . . . , denote the loci of the head 2A, B1, B2, B3, . . . , the loci of the head 2B; and a1, a2, a3, . . . , the track loci of the field tracks recorded by the recording head or heads having the same azimuth angle as that of the heads 2A and 2B. For a first field, in order to adjust the head locus A1 to the track locus a1, the head 2A must be continuously shifted to an extent from 0 to +0.5 TP within the scanning range of the first field. For a second field, in order to adjust the head locus B1 to the track locus a1, the head 2B must be continuously shifted from +1.5 to +2 TP within the scanning range of the second field. For a third field, to adjust the head locus A2 to the next track locus a2, the head 2A must be continuously shifted from +1 TP to +1.5 TP within the scanning range of the third field. For a fourth field, to adjust the head locus B2 to the track locus a3, the head 2B must be continuously shifted from +0.5 TP to +1 TP within the scanning range of the fourth field. The above-stated processes are repeated thereafter in a cycle of four fields. The pattern signal presented by FIG. 3(g) is arranged to satisfactorily carry out the shifting operation required for the heads 2A and 2B.
While the above description covers the 1.5 times increased speed reproduction, by way of example, the pattern signal generating circuit 15 can be arranged to produce any other pattern signal required in controlling the heads 2A and 2B for reproduction at any desired speed other than the speed increased by 1.5 times.
The pattern signal which is thus obtained from the pattern signal generating circuit 15 is supplied to the conversion element driving circuit 16. The driving circuit 16 then drives the electrical-to-mechanical conversion elements 3A and 3B according to the pattern signal to shift the heads 2A and 2B to the recording tracks to be reproduced. The conventional device thus has been arranged on the above-stated operating principle to obtain a noiseless reproduction of a video signal by virtue of the pattern signal for driving the shifting means such as the electrical-to-mechanical conversion elements, etc. However, the conventional rotating head type reproducing apparatus exhibits the following shortcomings:
Today there is a general tendency to abolish the conventional use of the CTL signal for VTR's. Should the VTR no longer use the CTL signal, the conventional pattern signal generating device, which indispensably uses the CTL signal, in generating a pattern signal would no longer be applicable to a VTR.
Further, some tracking error is inevitable in having a noiseless picture reproduced at a varied speed in the above-stated manner. To correct the tracking error, there has been practiced a method in which: The above-stated CTL signal is reproduced during a reproducing operation to detect thereby a tracking error; and the tape moving means, such as the capstan or the above-stated shifting means, is controlled accordingly. However, this method requires a long period of time for tracking. Particularly, in the event of slow motion reproduction where the tape is allowed to travel at a low speed, the time interval at which the CTL signal is reproduced results in an excessively long period of time for tracking. Further, this method makes it impossible to do tracking in the event of still picture reproduction.
Therefore, to make a tracking error signal always obtainable, it is conceivable to use a pilot signal such as the one described in the foregoing for tracking during varied speed reproduction. In carrying out varied speed reproduction, however, every recording track is not reproduced once one after another in the recorded sequence. Therefore, the frequency (or kind) of the pilot signal superimposed on the reproduced track cannot be discriminated. It is, therefore, also impossible to discern the frequency of the pilot signal obtained from adjacent tracks. It has been thus impossible to perform tracking by means of a pilot signal in carrying out varied speed reproduction.
Another problem of the prior art apparatus resides in that: The outputs of the counters 28, 20 and 22 as shown in FIGS. 3(c), 3(e) and 3(f) are not linear. If the number of pulses of the capstan FG signal to be produced per unit of time is infinitely large and the oscillation frequency of the oscillator 27 is extremely high, the outputs could be obtained in the waveforms as represented by FIGS. 3(a)-3(g). In actuality, however, the structural arrangement of the capstan 10, resulting from efforts to reduce the size of the apparatus, limits the number of pulses of the capstan FG signal generated per unit field (or a period during which the record bearing medium is moved to an extent corresponding to 1 TP) to a number from several to then odd pulses. Accordingly, the outputs of the counter 20 and 22 include small stepwise variations. Particularly, in cases where the record bearing medium is moved at a low speed during slow motion reproduction or the like, the number of pulses of the capstan FG signal generated per unit time becomes extremely small and, under such a condition, it is sometimes counted only two or three times per turn of the rotating head.
Therefore, depending on the capstan FG signal and the generation phase of the pulse signal produced by the oscillator 27, the pattern signals produced from the counters 20, 22 and 28 might have their phase deviating from the above-stated timing signal. Generally, this phase deviation increases, accordingly, as the number of pulses in the pulse signal decreases. Therefore, there is some probability that the fixed pattern signal obtained from the D/A converter 23 and the still pattern signal obtained from the D/A converter 29 deviate in phase from each other. Then, the driving pattern signal, obtained through addition, would be useless immediately after generation of the above-stated timing signal, i.e. immediately after switch-over from one head to the other. Accordingly, in that event, the opertaion of the electrical-to-mechanical conversion element becomes unstable before or after the head switch-over.
A further problem of the prior art apparatus lies in that: Generally, the shifting means, which is typically represented by the electrical-to-mechanical conversion element, is incapable of following a sudden change in a driving voltage and thus brings about a ringing phenomenon. FIGS. 5(a) and 5(b) show the relation of the driving voltage impressed on the shifting means to the displacement actually attained. As shown, when the driving voltage falls (or rises) at a point x, some vibration takes place for a while thereafter before the desired displacement is achieved. Under such a condition, it is nearly impossible to achieve good displacement or shifting control over the rotating head. In the event of a rise or fall accompanied by a large change in level, the period of such ringing lengthens to make the control nearly impossible. Therefore, with the driving voltage arranged to be supplied to the shifting means according to the pattern signal, such as the one represented by FIG. 3(g), the level of the driving voltage suddenly changes at the time of switch-over from one field to another. As a result, it has been nearly impossible to have the displacement of the rotating head satisfactorily controlled for the first half-portion of each field.
To solve the above-stated ringing problem, therefore, it has been generally practiced to have the pattern signal, which is generated at the pattern signal generating circuit 15, filtered by a low-pass filter (hereinafter called LPF) which is disposed within the conversion element driving circuit 16 before the pattern signal is applied to the electrical-to-mechanical conversion elements 3A and 3B. However, in the event of a rise or fall accompanied by a large change level, the ringing problem still arises as the cut-off frequency of the LPF cannot be lowered too much.
Another method for solving this problem is conceivable, for example, on the basis of the fact that the pattern signal is not necessary for controlling the head 2A during the periods corresponding to B1, B2, . . . , shown in FIGS. 3(a)-3(g) and, conversely, the pattern signal is not required for controlling the other head 2B during the periods corresponding to A1, A2, . . . . In this method, a fixed pattern signal for shifting the head 2A and another fixed pattern signal for shifting the head 2B are formed separately from each other and are used in such a manner that there takes place no sudden level change in the pattern signal immediately before the effective period of control over each of these heads. However, adoption of this method doubles the scale of circuit arrangement and is not desirable as it would result in complex circuitry.